The present invention relates, in general, to a hydraulic-mechanical tensioner.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
German patent publication DE 41 24 500 A1 describes a hydraulic tensioner having a cylinder in which a piston is able to move back and forth and bounds a pressure chamber that contains hydraulic fluid. The pressure chamber is hereby separated from a circular-ring shaped interior space between the outer cylinder wall and an inner wall of an enclosing housing. The cylinder-distal end of the piston is connected indirectly to a tension roller supported on a traction member, such as a belt of the traction drive. Tensioners of this type are oftentimes used in traction drives of internal combustion engines for driving aggregates, such as water pump, steering pump, air-conditioner compressor or generator.
During operation, the tensioner heats up like other components of the internal combustion engine. When the internal combustion engine is at a rest, the components are able to cool down. In the case of the tensioner, the pressure chamber decreases hereby in volume during the cool-down phase so that hydraulic fluid and air is being drawn in completely from a leakage gap located between the piston and the cylinder resulting in so-called lost motion. Lost motion causes faulty operation of the tensioner because of the underpressure in the pressure chamber of the cylinder, when the internal combustion engine is at rest. The underpressure is insufficient to open the one-way valve but, rather, draws the contained hydraulic fluid in the leakage gap. Once the leakage gap is emptied, air migrates from the interior space of the housing via the leakage gap into the pressure space to trigger the afore-mentioned lost motion of the tensioner.
Thus, the presence of trapped air in the pressure chamber during operation of the internal combustion engine, has an adverse effect on the operation of the tensioner because compressible air does not permit a defined position of the tensioner and thus of the tension roller, so that the biasing force of the traction member is adversely impacted as the tension roller is not properly supported upon the traction member. In addition, noise develops and slippage of the traction member is experienced resulting in increased wear.
It would therefore be desirable and advantageous to provide an improved hydraulic tensioner to obviate prior art shortcomings and to operate reliably regardless of any temperature impacts.